A diaphragm, and a loudspeaker unit using the same, capable of providing better low-tone characteristics than conventional ones. A loudspeaker unit includes a base frame and a cover plate joined to each other to provide a flat casing. Arranged in the casing are a diaphragm and a drive mechanism for vibrating the diaphragm. The diaphragm consists of a rectangular area and a pair of semi-elliptic areas, spreading rightward and leftward from the rectangular area, and has an elliptic central portion having its major axis in the longitudinal direction of the diaphragm and a peripheral portion spreading around the central portion. The central portion includes an adhesive layer for fixing a coil, laid on a central area of a diaphragm body, and is formed thicker than the peripheral portion.